Solid fuel is suitably used as fuel for, for example, thermal power generation or the like.
At present, bituminous coal is used as fuel for thermal power generation. However, the output of bituminous coal has been increasing year after year and there is a concern about exhaustion of bituminous coal. Accordingly, effective use of low-rank coal serving as an alternative to bituminous coal is an urgent issue.
Low-rank coal has a low calorific value and a spontaneous combustion property and hence applications of low-rank coal are limited. As means for effectively using low-rank coal, an upgraded brown coal process (hereafter, referred to as UBC process) has been used. To date, several processes for improving low-rank coal have been developed. However, it is difficult to practically use most of these processes because of high temperature or high pressure processing condition, which results in high apparatus cost, or low-rank coal is chemically changed to produce waste water containing a large amount of pyrolytic substances, which results in high waste-water treatment cost.
The applicant of the present invention disclosed such a UBC process in which low-rank coal is used as a raw material and an oil mixture containing a heavy oil and a solvent oil is made to be present in pores of the low-rank coal to thereby provide a solid fuel in which the content of the heavy oil in weight percentage with respect to the coal having been dewatered is 0.5% to 30% (Patent Literature 1). In the solid fuel of Patent Literature 1, water in pores of low-rank coal is removed and a heavy oil is made to adhere to the inner surfaces of the pores to cover active sites. Thus, spontaneous combustion of low-rank coal is suppressed and a heavy oil is contained in low-rank coal to achieve a high calorie. By dissolving a heavy oil in a solvent oil to achieve a low viscosity, pores are sufficiently impregnated with the heavy oil and, as a result, active sites in the pores are covered and a high calorie is achieved.
Improved-coal powder is scarcely used without being processed, as fuel. In general, improved-coal powder is compacted into briquettes and the briquettes are transported to a consumption site (for example, a thermal power plant or the like). When such briquettes have a low strength, the briquettes are cracked or produce powder during transportation or during loading/unloading operations. Thus, in addition to loss of a portion of the product, there is a concern about the increase in the probability of spontaneous combustion. Accordingly, briquettes are required to have a high strength.
In addition, to reduce production cost and to reduce environmental load, the proportion of components added from the outside is desirably made as low as possible or made zero.
Patent Literature 1: Japanese Patent No. 2776278